1. Field of the Invention
The present invention relates to a magnetic field alternation detecting apparatus for detecting that a magnetic member approaches the apparatus, based on an induced electromotive voltage generated by causing alternation of the magnetic flux density of a coil arranged to link magnetic fluxes by moving the magnetic member in a magnetic field.
2. Description of the Related Art
As a reference including descriptions associated with the apparatus of the present invention, the following Japanese-translated reference is known:
W. Leavens & N. Mencer, "Introduction of Car Electronics", Ed. Texas Instruments Learning Center
In general, as an apparatus for detecting the rotational speed or the rotational frequency of a rotating member, a magnetic field alternation apparatus serving as a rotation sensor (e.g. deposition sensor) or a rotational frequency counter is often used. FIG. 9 shows an arrangement of a main part of a magnetic member sensor of this magnetic field alternation apparatus. In FIG. 9, an axially magnetized magnet 2 is attached to one end of a ferromagnetic core (i.e., yoke) 1, a coil 3 is wound around the shaft portion of the ferromagnetic core 1 directly or via a bobbin (not shown), and such an assembly is stored in a case 4, thus constituting a magnetic member sensor. When a magnetic field is disturbed by a rod-like ferromagnetic member 5, which is axially supported by a rotational shaft O, and is approaching the other end of the ferromagnetic core 1, an alternation in magnetic fluxes linking coil 3 is obtained as an electromotive voltage from output terminals 3a and 3b of coil 3 on the basis of a magnetic field distribution which changes as time elapses. The magnetic field alternation detecting apparatus is arranged to be able to supply the electromotive voltage to a signal processing unit (not shown) via these output terminals 3a and 3b.
In this prior art arrangement, the electromotive voltage obtained from the output terminals 3a and 3b of the coil 3 is determined by the alternation amount of the magnetic fluxes and the number of turns of the coil. However, in general, since the shape and size of this apparatus are limited according to a rotation detection object, i.e., the ferromagnetic member 5, as a means for increasing the electromotive voltage, a coil having a small wire diameter is adopted, and the number of turns of the coil is increased.
However, in the above-mentioned detecting apparatus, in order to manufacture a coil in the magnetic member sensor as its main part, a conductor wire having a small diameter i.e., a thin conductor wire) must be wound around a core a large number of times. However, the manufacture of such a coil requires considerable amounts of labor and time. Also, it is difficult for a coil manufactured as described above to assure an anti-shock strength and to assure stability of coupling of lead wires between the output terminals and an external connection portion. More specifically, such wires of the coil are disconnected unless a tension generated upon a winding of the wire is controlled and adjusted very carefully. Even after the coil is completed, the wires may often be disconnected by an external "mechanical shock" or a "heat shock" caused by a change in temperature during processing. In addition, slight corrosion of a connection portion with the coil may cause disconnection.
The above-mentioned drawbacks occur due to a structural problem, while the following problems may also be posed from an electrical point of view.
More specifically, an electronic device itself such as a sensor of this type is very sensitive to induction of an external electromagnetic interference (EMI) since the winding impedance of the coil is increased. Since the line-to-line capacity is increased, the output itself is not proportional to the number of turns.
In an apparatus of this type, since a detection unit and a signal processing unit are arranged to be separated from each other, these units are connected via lead wires having a required length. Therefore, especially in the magnetic member sensor having a large number of turns, the lead wires themselves undesirably serve as an antenna. As a result, unnecessary EMI enters an electrical output. In addition, the distributed capacity of a receiving apparatus including that of the lead wires leads to a decrease in output level.